The present invention relates to a mass spectrometer and a method of mass spectrometry.
With the decoding of the 20-30,000 genes that compose the human genome, emphasis has switched to the identification of the translated gene products that comprise the proteome. Mass spectrometry has firmly established itself as the primary technique for identifying proteins due to its unparalleled speed, sensitivity and specificity. Strategies can involve either analysis of the intact protein or more commonly digestion of the protein using a specific protease that cleaves at predictable residues along the peptide backbone. This provides smaller stretches of peptide sequence that are more amenable to analysis via mass spectrometry.
A mass spectrometer comprising an Electrospray Ionisation (“ESI”) ion source interfaced to a tandem mass spectrometer has a particularly high degree of specificity and sensitivity. A complex digest mixture may be separated by microcapillary liquid chromatography with on-line mass spectral detection using automated acquisition modes whereby MS and MS/MS spectra may be collected in a data dependant manner. This information may then be used to search databases directly for matching sequences. This may lead to identification of the parent protein especially if the protein is present at low endogenous concentrations. However, often the limiting factor for identification of a protein is not the quality of the MS/MS mass spectrum produced, but rather is the initial recognition of multiply charged peptide parent or precursor ions in the MS mode. This is often due to the relatively high level of largely singly charged background ions emitted by the ion source and which appear in the resulting mass spectrum.
It would therefore be desirable to be able to recognise more easily multiply charged analyte ions of interest which are present in a mixture of ions which may comprise a significant proportion of singly charged background ions.